[PATCH v3 3/4] crypto: exynos - Reseed PRNG after generating 2^16 random bytes
flat view
STALE3235d
REVIEWED: 1 (0M)
From: Łukasz Stelmach <l.stelmach@samsung.com>
Date: 2017-12-12 16:37:24
Also in:
linux-samsung-soc, lkml
Subsystem:
crypto api, the rest · Maintainers:
Herbert Xu, "David S. Miller", Linus Torvalds
Status: Reviewed.
Revision v3 (3 revisions archived); 1 review trailer.
Revisions (3)
- v1
- v2 [diff against previous]
- v3 current [diff against previous]
Reseed PRNG after reading 65 kB of randomness. Although this may reduce performance, in most cases the loss is not noticeable. Also the time based threshold for reseeding is changed to one second. Reseeding is performed whenever either limit is exceeded. Reseeding of a PRNG does not increase entropy, but it helps preventing backtracking the internal state of the device from its output sequence, and hence, prevents potential attacker from predicting numbers to be generated. Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com> Reviewed-by: Stephan Mueller <redacted> --- drivers/crypto/exynos-rng.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/drivers/crypto/exynos-rng.c b/drivers/crypto/exynos-rng.c
index dcdd444d0b3b..825ed7bfd881 100644
--- a/drivers/crypto/exynos-rng.c
+++ b/drivers/crypto/exynos-rng.c@@ -55,12 +55,14 @@ enum exynos_prng_type { }; /* - * Driver re-seeds itself with generated random numbers to increase - * the randomness. + * Driver re-seeds itself with generated random numbers to hinder + * backtracking of the original seed. * * Time for next re-seed in ms. */ -#define EXYNOS_RNG_RESEED_TIME 100 +#define EXYNOS_RNG_RESEED_TIME 1000 +#define EXYNOS_RNG_RESEED_BYTES 65536 + /* * In polling mode, do not wait infinitely for the engine to finish the work. */
@@ -82,6 +84,8 @@ struct exynos_rng_dev { unsigned int seed_save_len; /* Time of last seeding in jiffies */ unsigned long last_seeding; + /* Bytes generated since last seeding */ + unsigned long bytes_seeding; }; static struct exynos_rng_dev *exynos_rng_dev;
@@ -126,6 +130,7 @@ static int exynos_rng_set_seed(struct exynos_rng_dev *rng, } rng->last_seeding = jiffies; + rng->bytes_seeding = 0; return 0; }
@@ -164,6 +169,7 @@ static int exynos_rng_get_random(struct exynos_rng_dev *rng, EXYNOS_RNG_STATUS); *read = min_t(size_t, dlen, EXYNOS_RNG_SEED_SIZE); memcpy_fromio(dst, rng->mem + EXYNOS_RNG_OUT_BASE, *read); + rng->bytes_seeding += *read; return 0; }
@@ -177,7 +183,8 @@ static void exynos_rng_reseed(struct exynos_rng_dev *rng) unsigned int read = 0; u8 seed[EXYNOS_RNG_SEED_SIZE]; - if (time_before(now, next_seeding)) + if (time_before(now, next_seeding) && + rng->bytes_seeding < EXYNOS_RNG_RESEED_BYTES) return; if (exynos_rng_get_random(rng, seed, sizeof(seed), &read))
--
2.11.0